RE: ICU4C proposal: Formalizing the ustdio format specifiers

> However, one uses the same format string for printf and for scanf. In
> scanf, the results are dumped into variables of the specified type,
> with no error checking. If they are not the right type, bad things
> happen.
> 
> Mark

Again, isn't this just the same picture, just backwards?

Here, I assume, from the format string we identify all patterns that are format specifiers, and those that are not. Those that are not identify the delimiters. Using the list of delimiters, get the set of substrings that represent the values to be converted from strings to 'whatever' from the source buffer. The set of format specifiers prescribe the format-types. The format-values here are strings, so set the type-id for the opaque data types of the slots to 'cstring', chain them in appropriate order, execute the context. The destination value type for each is that prescribed by format-type. After you have performed the conversion you check to see if the output type matches the temporary, or has a conversion between the two. If there is a conversion, perform it. Otherwise error. The opaque data type would look something like the following quick hack:

struct context
{
  // list of conversions to do...
  struct node
  {
    // used for dest in printf, source in scanf
    char sb[64]; // off stack
    char* hb;    // off heap if necessary
    char* ptr_to_which; // ptr to above

    // used as source in printf, as dest in scanf
    typedef union
    {
      char* sval;
      int8_t i8;
      int16_t i16;
      int32_t i32;
      int64_t i64;
      double  d;
      float   s;
      // others...
    } glob_t;
    glob_t gt;

    // descriptor
    typedef enum
    {
      one for each above;
    } desc_t;
    desc_t ty;

  } * exec_list;
  size_t list_len; // how many conversions
}

For each of the nodes and destination values in a scanf, a dynamic data conversion is performed. If no reasonable conversion exists, then it throws an exception, which you trap, and do the normal C thing.

I would think it would work something like that. Yes, no? I have not written such a beast, so that would be my best guess.

Bob Buck